A bipolar transistor and a metal-oxide-silicon (MOS) transistor are types of semiconductor devices. The bipolar transistor is preferred to the MOS transistor because the bipolar transistor operates at a faster switching speed in a digital integrated circuit. The bipolar transistor also has a greater amount of driving current per unit area when compared with the MOS transistor.
FIG. 1 illustrates a cross-sectional view of a conventional NPN bipolar transistor. As depicted in FIG. 1, a collector region 10 of a high-concentration n type impurity (hereinafter, ‘n+ type’) is formed in a bottom portion of a substrate 15 of a low-concentration n type impurity (hereinafter, ‘n31  type’). In the process of forming the n+ type collector region 10, the substrate 15 becomes an n− type collection region.
Typically, p type impurities are implanted into and activated in the n− type collector region 15 to form a p type base region 20. As well, n+ type impurities are usually implanted into and activated in a predetermined portion of the p-type base region 20 to form an n+ type emitter region 25. After forming the emitter region 25, an insulating layer 30 is often formed to cover the tops of the base region 20 and the emitter region 25. Next, the insulating layer 30 is partially etched to expose predetermined portions of the base region 20 and the emitter region 25. After etching the insulating layer 30, a base electrode 35B and an emitter electrode 35E are formed to contact the exposed portions of the base region 20 and the emitter region 25. Also, a collector electrode 35C is usually formed at the bottom of the high-concentration n-type collector region 10 using any method known in the art.
However, it is known that a “Kirk effect” (where the effective base width of a bipolar transistor extends to a collector region) is caused in the bipolar transistor of FIG. 1 when a current higher than a predetermined level is injected into the bipolar transistor. The occurrence of the Kirk effect results in an increase in the effective base width and a reduction in a current gain of the transistor. Also, the number of carriers is increased in the effective base region, thereby lowering the operational speed of the bipolar transistor.